Notions about human stem cells.

From an analysis of the known red cell and granulocytic cell turnover rates in the peripheral blood, absolute bone marrow cellularity of man, mitotic index of human marrow, tritiated thymidine labeling index of human marrow, the proportional distribution of the different cell types in the bone marrow of man and the measured DNA synthesis time of differentiated cell lines in human bone marrow, the flux of stem cells into the differentiated compartment has been calculated. By assuming that the same fraction of committed stem cells of man and mouse are in DNA synthesis, the number of cells in DNA synthesis and the size of the stem cell compartments have been calculated. The calculation gives an abundance of stem cells in human marrow that is much larger than that estimated by in vitro culture techniques. On the basis of the calculations, it is hypothesized that the pool of cells preceding the cytologically differentiated pools may consist of self replicating committed stem cells and differentiated cells undergoing amplification that have not developed cytologic markers characteristics of differentiated cells.